Seroney,
given that ythe conditions are anaerobic and pH and temperature are right is it possible you are having any contaminants in the system? If not, maybe you will get better results using cow manure.
As already mentioned acids like HCl, H2SO4, citric acid, sulfamic acid etc. won't attack the silica.
HF will work and also ammonium bifluoride. A mixture of HNO3 and ammonium bifluoride (2-3% ammonium bifluoride in 30% HNO3 aq.) is an effective alternative. A "milder" variant is 2-3% ammonium...
